Dicyandiamide



Nov. 17, 1959 R. NEUBNER ETAL 2,913,461

PROCESS FOR THE PRODUCTION OF MELAMINE Filed Jan. 16, 1957 K v i i 1 I a311- f .3

mcvmnmmm: 720 15 J a A 3 Q m 3 i0 19 1 MELAMINE 6 i F l INVENTORS.ROLFNEUBNER mzvzmmmmm ATTORNEY5 -products such asmelam, melem, rnelloneetc.

rR-ROGESS; FOR-,THE PRODUCTION Rolf Neubner,"Berlin-Zehlendorf, andFranz Langehenke,

N eubner v,ApplicationJanuary 16,,1957, Serial No. 634,454 Claimspriority, application Germany December '15, 1956 '4 Claims. (Cl.260-2495) The present'invention relates to a process of producingmelamine aswell astoa combined device for carrying out the process.

It iskn'own to produce melamine by heating dicyandiamide inliquid-ammonia underpressure. Allknown processes, however, possessconsiderable disadvantage. Either'they work .discontinuously orincomplete trans formation of the raw materials which are applied to thereaction vessel takes place there. Certain other processes createconditions favouring undesirable subsidiary reactions which formhighermolecular insoluble condensation If the transformation is eflected inthe gaseous phase, also as hitherto, temperatures and pressuresof such adegree-become necessary that therequiredexpenditure of energymakesthe-process considerably more expensive and the question ofmaterials plays a b-y no means insignificant part involving furtherdisadvantages and-increase in cost.

All-these disadvantages such as discontinuous operation,incompletetransformation, disproportionately high energy expense, -costincrease due tospecial materials for the equipment, and so on, areeliminated in the process according to the present invention. Apparatushas furthermore been designed in which the process according -to theinvention may'becarried out in an especially advantageous manner.

The process according to -'the inventionconsists essentially in thefactthat the-transformation of dicyandiamide into melamine takes placein a solution-of dicyandiamide. in-pure liquid; ammonia, theconcentration of the solution lying betweenabout -44 andabout 97 percent by weight dicyandiamide, this percentagebeing calculated on the sumtotal of dicyandiarnide plus ammonia; furthermore, thetransformationtakes-places at temperatures above the critical temperature 'of thepureammonia and below the melting point-of the pure dicyandiamide in areaction vesselunder a pressure dependent, at any given instant, on theconcentration of-the dicyandiamide-ammonia-solution and on the=reactiontemperature, which pressure may'be situatedbetween'41 atmospheres andthe critical pressure of pure ammonia.

One of the particular characteristicsof the present invention' lies inthe-'factthat the charging of the reaction vessel with thedicyandiamide-ammonia solution and the discharging from the reactionvessel of the melamine produced, and of the excessammonia by itsevaporation,

takes place in acontinuous manner, during which process the solidmelamine precipitated in the transformation is led, on leaving thereaction vessel, in countercurrent to the pure liquid ammonia, for thepurpose of a likewise continuous separation by washing out from the hotsolution which still contains untransformed dicyandioperation.

2,913,461 Paten ed ,Nov- .17, .1 .,5

It is advantageous toemploy,saturatedtdicyandiamide solutions in pureliquid ammonia for the charging of the reaction vessel, in such a waythatat anv arbitrarily regulated reaction temperature the highestpossible dicyandiamide concentration in the dicyandiamide-am:

.. monia solution prevails under the lowest. possible reac- Dbrtmrind,Germany said Langehenke assignor to said tion pressure which, on keepingthe reaction temperature constant, will be constant. "In this way alsothe best possible space/time yield of the process is obtained. As alowerconcentrationlimit, therefore, a Weight percentage of about 44 partsdicyandiamide, and accordingly 56 parts ammonia, i.e. adicyandiamidepmmoniasolution saturated atnormal pressure, ispermissible; As, however, with this concentration, a thermally neutraldevelopment ofthe process is no"longer possible, no concentration, willgenerally be chosen which lies below about 55 partsdicyandiamide,andaccordingly about 45 parts ofammonia, percent byweight.

The upper concentration'limit of the reacting solutionin which theammoniacontent does not amount to more than a few parts percent byweight is situated around 97 parts of 'dicyandiamide, and accordinglyabout 3 partsof ammonia.

The reaction temperature employed aecording tothe invention lies betweenabout133 C. and about 205 C., advantageously" between and C., whilst thereaction pressures lie between about 41 atmospheresand about 112atmospheres advantageously between about 41 atmospheres and about 70atmospheres. The pressure of about '41 atmospheres was, measured at thehigh est dicyandiamide concentration, aflittle above"the critigiven-instant insuch a Way that the gas pressure in the reaction vesselremains constant.

According to a further feature of the'invention, the ammonia evaporatedand re-liquefied in the. reaction'vessel is;used, after manometricstorage, for automatically maintaining constant the ,dissolvedmattercontent of the solution in the reaction vessel, being cooled down. to atemperature of about 25 C. to about 30 C. and then employedfor chargingthe reaction vessel together with the initial dicyandiamide-ammoniasolution by admixing the latter by means of volumetric displacementunder the pressure existing in the reaction vessel and with simultaneouspre-heating ofthis initial solution.

The further procedure according to the. inventionj is to use; thereaction heat freed during the formationof melamine for the evaporationof excess ammonia in the reaction vessel, for-heating the pre-heatedinitial dicyandiamide-ammonia solution and for heatinglto reactiontemperature the pure liquid ammonia which is displaced by the settlingmelamine and flows in counterevaporation is avoided by maintaining thedicyandi-.

amide content in the initial solution at such level that the ammoniavapour pressure of the initial solution, at

the pre-heating temperature needed for the aforesaid thermally neutraltransformation, is lower than reaction pressure.

Similarly to the aforesaid introduction, it is proposed according to theinvention to let the discharge of the crystal mash obtained after thetransformation and during the countercurrent washing-out process andconsisting of solid melamine and pure liquid ammonia, take place withoutinterruption of the melamine separation, by means of volumetricdisplacement by liquid ammonia.

The melamine at first present as a crystal mash in pure liquid ammoniasubsequent to the transformation, is now separated from such ammonia'during further development of the process according to the invention byheating the crystal mash under ordinary pressure and suspended in acirculated stream of heated gaseous ammonia, the liquid ammonia beingevaporated and separated from the circulation as a gas while the drymelamine is in turn separated from the gas stream and discharged underordinary pressure.

According to a further conception of the present invention, thedicyandiamide, previous'to its employment for the production of thedicyandiamide-ammonia mash, is freed from adhering residual moisture andall impurities soluble in ammonia by cooling the introduceddicyandiamide under normal pressure by means of a dicyandiamide-ammoniasolution saturated at about 25 C. and by washing it with a puresaturated solution of this kind also under normal pressure. Then alsothe final dicyandiamide-ammonia solution for the production of melamine,obtained from the dicyandiamide-ammonia mash, will of course be equallyfree of residual moisture and all impurities. Such portions of thedicyandiamide solution in ammonia which overflow in the region wheredoses of dicyandiamide stock are added and which contain water and otherimpurities are now freed of ammonia and water by heating, after whichthe solid dicyandiamide obtained at first in an impure state isreintroduced to the process, together with initial dicyandiamide, afterpurification by re-crystallization.

According to the invention, the apparatus for carrying out the describedprocess consists essentially of a reactor having a tube suitablydisposed at its bottom and provided with an agitator shaft, the tubeserving as a suspension washer in which the separated melamine settles,and of a sediment container to which the melamine is conveyed and whichat first is filled with pure liquid ammonia to be displaced by themelamine and which then proceeds to the aforesaid reactor by way of thesuspension washer, flowing in counter-current. to the settling melamine.The apparatus further consists of a cooling arrangement connected to thereactor head by a duct, for the liquefaction of the ammonia evaporatedin the reactor, of a manometric pipe for the storage of the lique fiedammonia, as well as of an adjoining cooling device which is connected toa valve or sluice in which initially there is the dicyandiamide-ammoniastock solution, and of a heater and a mixingtube by way of which thedicyandiamide-ammonia stock solution, after volumetric displacement bythe liquefied stored ammonia, proceeds from the valve to the reactor.

The apparatus also includes a melamine valve which is subject, similarlyto the sediment container, to the pressure existing in the reactor andwhich at first is filled with pure liquid ammonia; a circulation pumpwhich takes liquid ammonia from the head of the melamine valve anddrives it towards the crystal mash in the sediment container; and animmersion tube through which the crystal mash proceeds to the melaminevalve when driven from the sediment container.

There is also provided a reservoir filled with pure liquid ammonia, aswell as a suspension dryer into which the melamine crystal mash ispushed through the melamine valve by the ammonia from the aforesaidreservoir, and in which there is arranged a heater (calorifer) providedwith a blower from which issues a circulating stream of heated ammoniagas. The melamine heatdried in suspension is then separated in a cyclonefrom the ammonia gas stream; this cyclone has a valve for the dischargeunder ordinary pressure of the final melamine product.

Furthermore, for carrying out the process according to the invention,the apparatus includes a gasometer adapted to receive the ammoniaobtained as a gas in the suspension dryer after it has passed a dustfilter, as well as a compressor in series with the gasometer, for there-liquefaction of the ammonia, and a fractionating column, apre-cooling device, and the reservoir already mentioned which receivesthe circulated ammonia used for the displacement of the melamine crystalmash through the melamine valve, a main cooler and a compression washingdevice being incorporated with the reservoir, the bottom discharge ofthis washing device draining into the'condensate container and flowingunderneath said fractionating column.

The apparatus also comprises a screw conveyer for charging with stockdicyandiamide, a container with agitator into which dicyandiamide isfirst led, a suspension washing device in'series with such agitator inwhich device the dicyandiamide-ammonia stock solution originating fromthe agitator container is washed in countercurrent to a pure saturatedsolution of the same kind under ordinary pressure, and also an overflowarrangement associated with the agitator, and a heated paddle screw towhich the solution from such overflow containing water and otherimpurities is conveyed in equal doses, and a gasometer already mentionedto-which inter alia the ammonia is applied, across a dustfilter, whichescapes from the screw in a gaseous condition.

Finally, the apparatus for the production of melamine includes also adissolving vessel into which the dicyandiamide flows from the suspensionwashing device, and a reservoir into which the dicyandiamide-ammoniamash proceeds from the dissolving vessel by way of an immersion tube,and ducts through which the ammonia displaced thereby from the reservoirproceeds into the said dissolving vessel and into a storage tank fordicyandiamide-containing liquid ammonia; and also a preliminarydicyandiamide valve into which, the dicyandiamide mash is pushed throughan immersion tube by means of liquid dicyandiamide-containing ammoniainduced into the storage tank which ammonia proceeds into such storagetank from the reservoir with the aidv of a pump; and a pressure tankinto which the dicyandiamide-containing ammonia escapes, which is firstcontained in the preliminary valve. In series with the aforesaidheatable pressure tank there is arranged, according to the invention,amain condenser situated above the ammonia reservoir in which condensedammonia evaporated in the pressure tank is re-liquefied to the extentrequired in the reservoir for displacing the melamide crystal mash fromthe melamine valve into the suspension dryer. Additionally the apparatusis equipped with a gas separator in which the residualdicyanadiamide-containing ammonia from such pressure tank is expanded,and after its expansion and therefore cooling, escapes into thepressure-less storage tank. 1

The apparatus for carrying out the process according to the inventionincludes such further essential parts as a dicyanadiamide valve intowhich the dicyandiamide mash is pushed from the preliminarydicyandiamide valve after switching the latters connection with thereservoir and the pressure tank over to the connection with the saiddicyandiamide valve, and a circulation pump by which the liquid ammoniaat first located in the dicyandiamide valve is compressed, in order todisplace the dicyandiamide mash, into the preliminary dicyandiamidevalve.

A particularly advantageous device for carrying out the processaccording to the invention consists of a combination of single devicesand circulatory apparatus which cqmbination. is composed of a,pressureless; equipment for floating, washing and dissolved;dicyandiamide. in liquid .;ammonia, ahigh-pressuresystem forthe completedissolution of the dicyandiamide still .floating in thedicyandiamide-ammonia stock and initial solution and for the continuoustransformation of the dicyandiamide dissolved in pure liquid ammonia,into solid melamine, in conjunction with adevicefor the continuoussupply and with- -drawal ofxpure liquid ammonia and a device for thecon- .1 tinuous withdrawal, cleaning and collecting of the producedmelamine, and also a pressure-less device for sepa- -rating;;anddryingthe solid melamine from liquid am- ,moniaand, ammonia gas, and anammonia circulation system extending over all parts of the. combineddevice, with heating, cooling, compressing andv expanding as well ascleaningdevices and circulation pumps.

The;pressureless input equipment for cleansing, ;floating and dissolvingthe dicyandiamide in liquid ammonia consists-of a container withagitator mechanism, a suspension washing device, a dissolving vesselwith immersion;;tube, areservoir for the dicyandiamide mash as well as astorage tank for dicya-ndiamide-containing liquid am- -monia and thenecessary. communication ducts as well as feeding andremoval elements;the high-pressure system consistsof two dicyandiamide valves, a heaterand mixing;.,tube,, a reactor withsuspensionwashing device and sedimentcontainer connected in series, a melamine valve andof cooling devices.The equipment for separating the .solid melamine finally, consists of asuspension dryerwith ;heater-and;cyclone, and the ammonia circulationsystem consists of a gasometer, a compressor for the liquefaction ,ofammonia and of circulation pumps for conveying ammonia and ammoniasolutions as well as of a fractionatingcolumn, cooling devices, acompression washing de- .vice, the reservoir, a pressure tank,thegas-separator and of, communication ducts.

The valve devices, i.e.'the melamine valve, the preliminarydicyandiamidevalve, the dicyandiamide valve and the reservoir for. the dicyandiamidemash may, ac- .cording to a further conception .of the presentinvention, be formed as twin containers whereby the operation of .suchvalve devices may advantageously also take place in a continuous manner.

In particular, the following .further .points are. explained:

Melamine results from dicyandiamide with favourable yield and a highpurity when dicyandiamide is dissolved .in liquid ammonia and thesolution is heated under pres- ,sure. The transformation however takesplace at sufiicient speed onlyat temperatures above 133 C., this beingthe critical temperature of the pure liquid ammonia.

The critical temperature andxthe critical pressure of the-pure liquidammonia are altered by substances such .as dicyandiamide when this isadded to the liquid ammonia and dissolved therein, according to knownformulae regarding increased boiling point and decrease of vapourpressure of such solutions,.when compared with the cor- .responding puresolvent ammonia.

The high solubility of the dicyandiamide in liquid ammonia according towhich for instance at minus 25 C. and under atmospheric pressure theremay be dissolved, in 100 parts by weight of ammonia, as much as about76.5 parts by weight of dicyandiamide, permits even the production ofammonia-dicyandiamide solutions of e.g. about 80 parts of dicyandiamide,and accordingly about 20 parts of ammonia, by weight percentage, whichsolutions are still in a liquid state at temperatures of over 186 C.

Atthe latter temperature, however, a rapid transforma- "tion intomelamine takes place, the melamine produced precipitating from thesaturated hot solution. The con tent of dissolved substances in thesolution diminishes according to this precipitation of solid melamine.In order to maintain the liquid state at the high super-critical te prature,- h ;precipitate ;me ammeimusthowevena 13 11 136 ,b r bsti ute rfresh; eyan iamid t-M l i mine is'then continuously obtained imasaturatedzsolution whose concentration must be the higher. thegreatenthe temperature of the :reacting solution.

To obtain melamine from the solution at super-critical temperatures of'the pure liquid ammonia, it is -ther efore essential to separate thesolid melamine from the saturated solutions and to maintain constant, atthesame time, a high dicyandiamide concentration. In the continuous modeof operation required for this, conditions are made additionallydifiicult by the need of'having to remove melamine continuously from thereactor and ;to separate the melamine from thesaturatedsolution-whilemaintaining at the'same time theconstant highdicyandiamide concentration.

In the process vaccording to the invention-forcontinuous production ofmelamine from dicyandiamide-ammonia solutions in-the super-criticaltemperaturerange ofthe pure liquid ammonia, the continuous dischargefrom-,the reactor of'the melamine formed during the transformation and.the continuousseparation of suchrnelamine from the saturated.:dicyandiamide ammonia solution is madepossible, without at the sametime having to Withdraw such dicyandiamide-ammonia solution from thereactor and under. a constant reactionpressure, and'melamine washed withpure liquid ammonia is obtained without filtration. Further advantagesresult in the transformation itself when carried out according to theprocess of the present invention, and also during the making and thecontrolledfurnisliing according to the invention,'of the initialsolution which will be explained in more detail in the following.

According to theinvention, the procedure inparticular providesthatthemelamine crystallizing out in the reactor and quickly settling atthe bottom sinks from there into a vertically arranged tube located atthe bottom of-such reactor, which tube merges into a containerfilledwith pure liquid ammonia in which container the melamine willsettle. Thereby the settling melamine displaces from such container intothe reactor an amount of liquid-ammonia corresponding to its own volume,the displaced ammonia which flows in countercurrent to the melamine inthe direction of the reactor, being capable of separatingthe-dicyandiamide-containing solution from the melamine and completelycleaning'the melamine. The liquid column in the vertical tubecommunicating between reactor'and the separator and sediment containerarranged beneath it is kept gently rotating, e .g.- by a-vaneless shaft,in order to avoid laminarcurrents. The melamine isthereby completelyseparated, ina-suspended state, from theupper dicyandiamide-containingsolution and is moreover washed, in the lower part of the vertical tubewith pure liquid ammonia. The vertical tubewill in the following betermed the suspension washing device.

It has also been found that the suspension washing device may at thesame time serve as heat exchanger for a complete heat exchange betweenthe settling melamine and the upward-flowing pure liquid ammonia, themelamine thereby proceeding in a cooled state to the separating andsettling container, after having imparted its effective heat to theammonia which flows in the opposite direction.

In the accompanying drawing a device for'carrying out the processaccording to the invention is shown diagrammatically and partly insection. This device, according to the invention, firstly consists ofa-reactor 1 with the suspension washing device 2 connected inseries-with it, and the settling container- 3. The charge volume inreactor 1 including the suspension washing device 2 and the settlingcontainer 3 remains unaltered-during melamine formation. i

The discharge from the settling'container 3 of the crystal mashconsisting of melamine and pure liquid ammonia is also effected withoutvolume alterationand withoutinterruption ofmelamine separation by meansof I 7 a melamine sluice or valve 4 which is at first also filled withpure liquid ammonia. The ammonia is withdrawn from valve 4 by means of acirculation pump 5 and is pressed on the crystal mash in the settlingcontainer 3 in such a way that the crystal mash is pushed through anammersion tube 6 towards the melamine valve 4 which is also stillsubject to the pressure existing in reactor 1, in the suspension washingdevice 2 and the settling container 3. The settling container 3 is thenagain charged with pure liquid ammonia while the crystal mash is now inthe melamine valve 4.

This valve 4 containing the crystal mash is then separated, by means oflocking members, from settling container 3 and is connected with areservoir 7 for pure liquid ammonia in which there is ammonia at atemperature of about 30 C. at the corresponding boiling point-pressureof about 12 atmospheres. The crystal mash in the melamine valve is nowpushed, under the aforesaid pressure, into a suspension dryer 8 by meansof the liquid ammonia from reservoir 7, in which dryer the crystal mashis freed, under ordinary pressure, from liquid ammonia by means ofheated ammonia from the heater 9 which is provided with a blower. Thedry melamine is quantitatively separated in a cyclone 10 and in a dustfilter 11 while the ammonia evaporated in the suspension dryer 8 is ledas an excess gas through a gasometer 12, a compressor 13, afractionating column 14 for separating moisture possibly carried fromthe gasometer 12, and a preliminary cooler 15 arranged behind or abovesuch fractionating column 14, into a main cooler 16 from which theammonia by now re-liquefied returns by circulation to reservoir 7.

The residual gases accumulating in the main cooler 16 may be freed fromammonia in a compression washing device 17 by means of water and thenreleased upwardly. The ammonia water thereby obtained is again freedfrom ammonia in the condenser receiver 18 underneath the aforesaidfractionating column 14 by means of heating, such ammonia proceedinginto the fractionating column 14 while the water component is drawn offacross a levelregulator 19 and thereby expanded.

The continued removal from reactor 1, of the melamine produced by meansof the suspension washing device 2, causes the ammonia displaced by thesettling melamine to proceed from the settling container 3 into reactor1 in which, together with the ammonia introduced in the form of thedicyandiamide stock solution, it has to be evaporated in order tomaintain a constant dicyandiamide concentration in this reactor. Suchammonia evaporated in reactor 1 is condensed under reaction pressure,i.e. under the pressure prevailing in reactor 1, in the cooler 20connected in series, the condensation temperature being a reactivelydetermining factor for the reaction pressure in reactor 1. Thiscondensation temperature of the ammonia which is to be liquefied is verydifferent from the temperature of the dicyandiamideammonia solution inreactor 1, such difference depending on the dissolved-matter, i.e.dicyandiamide-content of such solution in reactor 1 at any given time.It has been found for instance that in reactor 1, at a temperature of182 C., there results an ammonia gas pressure of about 64 atmosphereswith a condensation temperature in cooler 20 of 101 C. This differencein temperature at a given time is a measure of the saturation of thesolution in reactor '1. It remains constant only if the saturation ofthe dicyandiamide solution in reactor 1 is kept constant by the additionof fresh dicyandiamide-ammonia stock solution in accordance with themelamine formation proceeding in reactor 1.

The dosed addition into reactor -1 of dicyandiamide in the form of thedicyandiamide-ammonia stock solution is possible according to knownpractices, e.g. by power regulation of a suitable pump which takes thedicyandiamideeammonia solution from a suitable pressure container.According to the invention, the dosed addition of thedicyandiamide-ammonia stock solution into reactor 1 may now take placewith automatic regulation and without such dosing pump in that theammonia reliquefied while still under reaction pressure is used to pusha dicyandiamide crystal mash from a dicyandiamide valve 21 through aheater or pre-heater 22 and a mixing tube 25 into reactor 1.Dicyandiamide crystal mash is here intended to mean a mash of excessdicyandiamide floated in a dicyandiamide-ammonia solution, the completedissolution of the dicyandiamide taking place here only within heater22, but prior to entry into reactor 1.

The ammonia liquefied in cooler 20 should be manometrically dammed in apressure tube 23, previous to its use for the displacement of thedicyandiamide crystal mash from the dicyandiamide valve 21, to obtainthe differential pressure required for such displacement, and should becooled in a further cooler 24 from its condensation temperature down toabout 26 to 30 C. in which process, by suitable regulation of thetemperature of such cooled ammonia,'it is possible to adjust its volumeaccurately to the volume of the displaced dicyandiamide crystal mash.

Self-regulation of dicyandiamide additions in the shape of the mash orof solution in liquid ammonia, which must correspond at any given timeto melamine production in order to maintain a constant saturation of thesolution in reactor 1, is effected by direct dependence, according tothe invention, of the amount of ammonia liquefied in cooler 20 on themelamine formation itself if, as is the case here, operation is effectedunder constant gas pressure. During increased melamine formation andprecipitation there takes place a correspondingly additional decrease ofthe dissolved matter content in the reacting solution which in turnresults in an increased vapor pressure of the solution. In maintaining aconstant gas pressure, there now takes place however an additionalammonia evaporation which is further increased by the fact that, owingto the increased melamine formation also the amount of the ammonia isincreased which is displaced by melamine from the separator and settlingcontainer 3 and flows into reactor 1.

A change in melamine formation under constant gas pressure depends onthe saturation and temperature of the dicyandiamide-ammonia stocksolution at which such stock solution is fed into reactor 1 if no use isto be made of the reactor walls for the passage of heat. In suchthermally neutral operation of reactor 1, the reaction heat producedduring melamine formation is just sufiicient to cover the amount of heatneeded to evaporate the ammonia in reactor 1 and to heat the pre-heateddicyandiamide stock solution and the ammonia displaced from settlingcontainer 3 by the melamine. The thermally neutral operation may also beachieved with different concentrations of the dicyandiamide stocksolutions fed into reactor 1 by accordingly regulating the inputtemperature of such stock solution into reactor 1. This temperature willhave to be kept the higher, the lower the dicyandiamide content of suchdicyandiamide-ammonia stock solution.

The pro-heating of the dicyandiamide-ammonia stock solution takes placein heater 22 in which a complete dissolution of still solid parts takesplaces when solution still containing such solid dicyandiamide areemployed. In order to avoid gas formation in heater 22 due to possibleammonia evaporation, there is to be maintained a maximum preheatingtemperature at which the vapor pressure of the dicyandiamide-ammom'astock solution has reached the gas pressure in reactor 1. In stocksolutions with a low dicyandiamide concentration, i.e. with about 30parts by weight of dicyandiamide for parts ammonia, such maximumpre-heating temperature is already reached before reaching'thepre-heating temperature needed for a thermally neutral operation.Thermally neutral operation is therefore dependent on a minimumdicyandiamide concentration in the dicyandiamide- I ammonia stocksolution while a thermally neutralmode of operation is safelyattainable, when highly concentrated-stock solutions with about 70 to240 parts by weight of dicyandiamide for 100 parts by weight of ammoniaare employed, by the mere regulation of the preheating temperature, inwhich case fluctuations of dicyandiamide concentration in such stocksolution may be compensated without danger of evaporation in heater 22.The advantages of a thermally neutral mode of operation in reactor].consist in the fact that the transformation vcan be carried out withoutthermal stress of the reactor wall and independently of the reactorsize.

-.In thiszway it is also possible to employ corrosion-proof found that.thepdicyandiamide valve 21 may be charged,

withoutintermptionof the self-regulating dosed addition of thedicyandiamide mash "as dicyandiamide-ammonia stock solution into reactor1, with fresh dicyandiamide mash by pressing the liquid ammonia whichwas used for the displacement of the .dicyandiarnide mash through heater22 towards reactor 1, from dicyandiamide .valve'21 into apreliminarydicyandiamide valve .27 by means of a circulation pump. 26 whichpreliminary valve is itself filled with fresh dicyandiamide mash. Thedicyandiamide mash is displaced therefore from the pre- 'liminarydicyandiamide valve 27 towards the dicyandiamide valve 21 while theliquid ammonia is-pumped by means of the circulation pump 26 from thedicyandiamide valve 21 towards the preliminary dicyandiamide valve 27.The preliminary dicyandiamide valve 27 is subsequently separated fromthe dicyandiamide valve 21 by closure members and connected in its upperpart to a pressure container 29 in which is containeddicyandiamide-containing ammonia and which is connected by thecommunication duct 30 to the main cooler 16 subjectto an ammoniapressure of about 7.5.to 12 atmospheres which main cooler is arranged inthe ammonia circulation system above reservoir 7 for liquid ammonia.Immersion tube 28in-the preliminary dicyandiamide valve 27 is in itsturn connected to storage container 31 for the dicyandiamide mash fromwhich such mash is displaced, 'by'means'of dicyandiamide-containingammonia, from the pressure-less storagetank 32 with the aid of a pump 33through immersion tube 28 towards the "preliminarydicyandiamide 'valve27 while the di-- cyandiamide-containing ammonia is displaced from suchpreliminary dicyandiamide valve27 towards pressure container 29 by thefreshly introduced mash. It was estab- -lished that-during the"displacement of" the dicyandiamide mash by liquid ammonia which takesplace twice according-to the process oftheyinvention','fromthe mashstorage container'31 towards the preliminary dicyandiamide valve 27 andfrom there towards the dicyandiamide valve "21,"on1y-insignificantmixing takes place at the contact surfacesof the dicyandiamide mash withthe liquid am-- monia which is why mechanical separation of the twomedia in the valves,;e.g.-by'a'freely movable plunger, is unnecessary.The reason why no appreciable mixing takes place is that the specificweight of liquid ammonia,

e.g. at. minus 27 C., is 0.60 while the specific weight ofadicyandiamide mash containing e.g. 48.2 parts of solid dicyandiamide,22.5 partsv ofv dissolved dicyandiamide and 29.3;parts;of1ammonia byweight,percentage,pat a temperature of minus. 25 C.,; amounts ,to 1.06.

.-; It-'was a1so established.- that theliquid; ammonia when 75 for thedisplacement of melamine. of liquid dicyandiamidecontainingammoniais-removed usedseveral times for the displacement of-the dicyandiamidemash subsequently contained about 5- to 15v parts by weight ofdicyandiamide for 100 parts by weight of ammonia. While on the one. handthere is no objectionto employing such dicyandiamide-containing ammoniafor producing the dicyandiamide mash, such dicyandiamide-containingammonia may not, on the other hand, beused for the discharge of melamineby melamine displacement and has to be freed of dicyandiamide previousto its being used for such purpose. For this reason, an amount ofammonia is evaporated from dicyandiamidecontaining ammonia in theheatable pressure container 29, under a pressure of about 7.5 to 12atmospheres and condensed in the main cooler 16 above the ammoniareservoir 7 being by now dicyandiamide-free ammonia, equal to the amountwithdrawn from such reservoir 7 The residual amount from the pressurecontainer 29 and expanded in a gas separator 34 whereby a further partof the ammonia evaporates which is being reliquefied above the gasometer,12 in a way already described while the dicyandiamide containingammonia, expansion-cooled to about minus 27 C., runs from the gasseparator 34towards the pressure-less storage tank 32.

The production of; the dicyandiamide mash under nor- .mal. pressure fromsolid dicyandiamide and from dicyandiamide-containing ammonia with aboiling vpoint of about minus 27 C. maytake place in a standardcontainer 36 with built-in stirring mechanism.

Theammonia hereby evaporated is fed to gasometer 12 for liquefaction.First there will result under normal --pressure.-a saturated solution ofdicyandiamide in ammonia which boils at minus C- and which is changedinto a dicyandiamidemash by further addition of solid dicyandiamide. Itwasfound'that this mode of operation mayadvantageously be combined witha quantita I tive dehydration and cleansing of the solid dicyandiamidethe water introduced with such dicyandiamide, e.g. in the shape ofanadhering residual moisture of 0.2 percent by weight being alreadyremoved before the solid dicyandiamide is changed into a mash.

The transformation ofdicyandiamide into melamine .takes placeawith thegreatest yields and highest purity in the absence of water.

Small amounts .of water up to about 5 percent:by.weight do not impairsuch transformation appreciably if the operation takes place under highammonia pressure. During the continuous trans- I formation in a reactorhowever even the residual moisture carried by the dicyandiamide causessuch an accumulation of water in the reactor that tolerable limitsaresurpassed within a short time as becomes evident from the followingexplanations:

During a mean period of e.g. 15 minutm in which dicyandiamide is presentfor the purpose of the transformation. into melamine, and with amoisture content in the solid dicyandiamide of 0.2 percent by weight 0.2kg. of

water accumulate in the reactor within 15 minutes for 100 kg. oftransformed dicyandiamide.

Within 24 hours therefore, no less than 4.8 kg. of water will becontained in the reactor for 100 kg. of dicyandiamide and an only justtolerable maximum limit will have been reached. The water will thenhowever be extractable from the reactor only together with the hot andconcentrated solution. But to withdraw the aqueous solution from thereactor and to prepare the hot saturated solution always involves losseswhich, owing to the separation of water according to the inventionalready in the preliminary mash,-may-be avoided. V

In order to separate such water during the preparation of thepreliminary mash, the solid dicyandiamide is fed in dosed additions intostirring container 36 vby a screw conveyor 35. The said stirringcontainer 36 is connected, together with the inlet from screw conveyor35, and

-.-.-.through a wdustnfilter- .37, with gasometer, 12, .through whichpossibly evaporated ammonia is re-liquefied. At

'the bottom of stirring container 36 there is arranged a containingliquid ammonia, the line leading to storage tank 32 being throttled bymeans of an automatic regulation device in such a way that the liquidammonia pro ceeds through dissolving vessel 39, and from there as asaturated dicyandiamide-ammonia solution through the suspension washingdevice 38, into the stirring container 36 up-to the overflow arrangement43, whereby it is dammed to that extent.

The non-pre-cooled dicyandiamide falls from the screw conveyor 35 intothe dicyandiamide-ammonia solution in stirring container 36 which issaturated at minus 25 C., the dicyandiamide also being cooled to minus25 C. by the evaporating ammonia, and proceeds, evenly distributed by astirring vane in such stirring container 36, into the suspension washingdevice 38 in which it settles and is thereby completely separated bywashing out in countercurrent from water and other impurities soluble inammonia. The washed dicyandiamide finally proceeds into dissolvingvessel 39 in which only an amount of dicyandiamide is dissolved by theammonia supplied over duct 41 as will result in a solution saturatedunder normal pressure and at minus 25 C. with about 76.5 parts ofdicyandiamide for 100 parts of ammonia by weight. This is equivalent toa dicyandiamide-ammonia solution con taining about 43 parts ofdicyandiamide and about 57 parts of ammonia percent by weight. Theundissolved dicyandiamide proceeds, together with saturateddicyandiamide-ammonia solution, in the form of a so-called dicyandiamidemash through an immersion tube 40 into storage container 31. Thecomposition, i. e. the proportion of undissolved dicyandiamide in theammonia solution saturated with dicyandiamide, is automatic if the dosedaddition of dicyandiamide is maintained constant. The dicyandiamide mashstill contains between about 18 and about 55 percent by weight ofundissolved dicyandiamide. v

If a dicyandiamide mash contains for instance 18 percent of undissolveddicyandiamide and therefore 82 per cent of dicyandiamide-ammoniasolution by weight which solution in turn consists as stated above, of43 percent of dicyandiamide and accordingly 7 percent of ammonia byweight, there results a composition, when related to the total quantityof undissolved and dissolved dicyandiamide plus ammonia, of 18 parts ofundissolved dicyandiamide, 353 parts of dissolved dicyandiamide and4.6.7 parts of ammonia by weight percentage. After complete dissolutionin the pro-heater or in heater 22 of the solid dicyandiamide stillpresent in the mash, the dicyandiamide-ammonia solution which is now ledinto reactor 1 is composed of a total amount of dissolved dicyandiamideof 53.3 percent and accordingly 46.7 percent of ammonia by weight. Acalculation carried out accordingly produces, with a dicyandiamide mashhaving for instance 55 percent of undissolved dicyandiamide andaccordingly 45 percent of dicyandiamide-ammonia solution by weight, acomposition which if related to "the total quantity of undissolved anddissolved dicyandiamide plus ammonia, amounts to 5 5 percent ofundissolved dicyandiamide, 19.4 percent of dissolved dicyandiamide and25.6 percent of ammonia by weight. After complete dissolution in thepre-heater or heater 22 there results accordingly in this case acomposition of the dicyandiamideammonia solution containing a totalamount of dissolved 12 dicyandiamide of 74.4 percent and accordingly25.6 percent of ammonia by weight.

In relation to 100 parts by weight of ammonia therefore, thedicyandiamide-ammonia stock solution contains a total dicyandiamideamount of between about 100 and about 300 parts by weight, andparticularly between about 114 and about 290 parts by weight.

The solution containing water and other impurities which forms in thestirring container 36 during cooling of the freshly introduced soliddicyandiamide is led in evenly dosed additions, with a water content ofup to about'S percent by weight, from the overflow arrangement 43 ofstirring container 36 to a heated paddle screw conveyor 44 and is heatedin the said paddle screw conveyor 44, such water being led, togetherwith the ammonia, through dust filter 45 to gasometer 12 as a gas whilethe separated impure solid dicyandiamide may be reintroduced into screwconveyor 35, after purification by recrystallization. The preliminarycleaning of dicyandiamide with liquid ammonia according to the inventionresults in an even mode of operation during the transformation of thedicyandiamide in reactor 1, as well as in the even production ofmelamine of the highest purity whereby the additional expenses necessaryfor the described preliminary cleaning of the dicyandiamide arejustified. The operation of the valve devices may, according to afurther conception of the present invention, also take place in acontinuous manner it the melamine valve 4, the preliminary dicyandiamidevalve 27, optionally also the dicyandiamide valve 21 and the storagecontainer 31 are constructed as double containers.

The following example serves to illustrate the invention but it is notintended to limit it thereto:

Example From 100 kg. of dicyandiamide and 41.6 kg. of liquid ammonia adicyandiamide mash is produced under normal pressure which contains 68.3kg. of undissolved dicyandiamide in a solution saturated at minus 25 C.and containing 31.7 kg. of dissolved dicyandiamide in 41.6 kg. ofammonia. Before entering reactor 1, the still undissolved dicyandiamidepart of the dicyandiamide mash is dissolved under a pressure of 64 at.in heater 22, pre-heated to a temperature of +115 C. and diluted in themixing tube 25 with 37.8 kg. of liquid ammonia from the suspensionwashing device 2, in such a way that the dicyandiamide-ammonia solutionwhen entering the reactor 1 contains 100 kg. of dicyandiamide and 79.4kg. of ammonia. The transformation in reactor 1 takes place at atemperature of 182 C. under a gas pressure of 64 at. the freed reactionheat being just about consumed in evaporating the introduced ammonia andin heating the pre-heated dicyandiamide-ammonia stock solution to theaforesaid reaction temperature.

The evaporated ammonia is condensed under reaction pressure in cooler20, 79.4 kg. of liquid ammonia being obtained at a temperature of 101 C.After further cooling of the liquefied ammonia to +28 in cooler 24 underoperational pressure, the liquefied ammonia has a volume of 132.5 litresand displaces the dicyandiamide mash also under reaction pressure fromthe dicyandiamide valve 21, the aforesaid quantities being re-fed intoreactor 1 across heater 22 and mixing tube 25.

-During the transformation, kg. of melamine ar produced from 100 kg. ofdicyandiamide and are continuously discharged from reactor 1,automatically washed in the suspension washing device 2, with 37.8 kg.of liquid ammonia flowing in opposite direction from the settlingcontainer, and cooled during such process, the volume of the liquidammonia being 63.6 litres at +30 C. and coinciding with the volume ofthe cooled melamine which is also 63.6 litres.

The melamine crystal mash separated in liquid ammonia at +30 C. isdisplaced at reaction pressure from sediment container 3 by pure liquidammonia which is taken from the melamine valve 4, by means ofcirculation pump *5.-.towardssuch melamine valve 4. from 3 which isunder reaction pressure,'by liquid ammonia taken from the reservoir 7towardsthe suspension dryer 8 under azpressure of 12 .at. and atetemperature of +30 C. in which dryer the crystal mash-is now freed fromthe adhering pure liquid ammonia under .normal pressure. To this end 100kg. of melamine are distributed, together with a residual 22.5 kg. ofliquid ammonia, in the suspension dryer 8 through anozzle and dried insuspension in an ammonia gas stream heated in heater 9. The melamine isseparated from the ammonia gas stream in cyclone 10 and in dust filter11 connected in seriesand discharged from such cyclone 10 as pure drymelamine with a purity of 99.7% and a yield of over 99 parts by weightpercent. The said yield is referred to the. amount of dicyandiamideintroduced into the process over the dicyandiamide valve 21. quantitycorresponds to the dicyandiamide originally supplied over screw conveyor35, less the proportion of dicyandiamide which is again discharged asdirty salt over paddle screw conveyor 44 and Whose amount depends on thegiven moisture content and the impurities in the dicyandiamide rawmaterial.

The preparation of the ammonia obtained in a gaseous state during thedrying operation is effected by uniting it first of all in gasometer 12with gaseous ammonia obtained during production of the dicyandiamidemash in stirring contaner 36, during the cleansing opera tion of thedicyandiamide in the heated paddle screw conveyor 44 or in dust filter45 and during expansion of liquid ammonia to normal pressure and coolingfrom +30 C. to approximately 30 C. in the gas separator 34, whichgasometer 12 serves for the reception of the obtained ammonia gas. Thetotal quantity of ammonia gas obtained here amounts to 48.7 kg., i.e.62.5 Nmfi, which are thereafter compressed in compressor 13 to apressure of 12 at. and obtained free from water as a pure gas in thesubsequent fractionating column 14 to be fed, across a preliminarycooler 15, to the main ammonia cooler 16 from which ammonia proceeds inliquid state at +30 C. to a reservoir 7 and is now ready for further usein the melamine valves 3 and 4.

The preparation of the liquid ammonia subject to reaction pressure whichafter displacement of the dicyandiamide mash is present in thedicyandiamide valve 21 is effected by first using the liquid ammonia todisplace, under reaction pressure and with the aid of circulation pump26, the dicyandiamide mash from the preliminary dicyandiamide valve 27towards the said dicyandiamide valve 21 whereupon thedicyandiamide-containing ammonia, after switching the preliminarydicyandiamide valve 27 from its connection with the dicyandiamide valve21 over to the connections with pressure container 29 on the one end andstorage container 31 for the dicyandiamide mash on the other hand, isnow led with the aid of pump 33 from storage tank 32 under a pressure of12 at. to storage container 31 and the dicyandiamide mash is therebydisplaced from the said storage container 31 across immersion tube 28towards the preliminary dicyandiamide valve 27, the ammonia with aslight dicyandiamide content being displaced at the same time from thesaid preliminary valve 27 towards pressure contaner 29, or escaping inthis direction, which container is also under a pressure of 12 at.

Thereafter there are 79.4 kg. of liquid ammonia in pressure container 29containing only a little dicyandiamide at a temperature of +28 C. ofwhich 67.8 kg. are expanded to normal pressure in which process 14 kg.of ammonia will evaporate and proceed to gasometer 12 while 53.8 kg. ofliquid ammonia are obtained at a temperature of minus 30 C. Of this,41.6 kg. are

This

consumed for the production of; the dicyandiamidemash .in'thecirculation through storagetank 32, storage container -31, dissolvingvessel 39, suspension washing device 38 and stirring container 36 and inreverse direction while the residual amount evaporates in cooling thesolid dicyandiamide introduced by screw conveyor 35 and is alsoevaporated inthe paddle screw conveyor 44 in preparing the aqueousdicyandiamide-ammonia so lution which overflows in stirring container36. The nonexpanded excess ammonia left in pressure container 29 isevaporated under a pressure of 12 at. in the said pressure container 29by heating and is recovered in main cooler 16 together with the gasesfrom gasometer 12 compressed in compressor 13, as dicyandiamide-freeliquid ammonia. The surplus gasified in pressure container 29 amounts to11.6kg. of ammonia-which, together with 48.7 kg. of ammonia gasescompressed in compressor 13 from gasometer 12, yields just therightamount requiredfor the valve operation with, regard to the melaminerforits displacement-from sedimentcontainer 3 with 37.8 kg., and for themelamine discharge from the melamine valve 4 into the suspension dryer 8as a crystal mash, with 22.5 kg. of ammonia.

The ammonia losses during the operation according to the inventionamount to less than 1.5 kg. of ammonia, related to a yield of kg. ofmelamine.

In comparison with hitherto disclosed transformations of dicyandiamideinto melamine from dicyandiamide-ammonia solutions at temperaturesbeyond the critical temperature of the pure liquid ammonia, it is seenthat, as a result of the automatic separation of the melamine from thehot saturated dicyandiamide-ammonia solution in the reactor on the onehand, and of the automatic washing out of the melamine in the suspensionwashing device with pure liquid ammonia with simultaneous cooling of themelamine on the other hand, it is possible to attain the followingdecisive advantages.

The dicyandiamide concentration in the reactor may be kept at a constantlevel by means of'evaporation of ammonia without impairing thetransformation.

The use of high dicyandiamide concentrations in thedicyandiamide-ammonia stock solution in the form of a dicyandiamide mashwith approx. 75 parts of dicyandiamide and 25 parts of ammonia by weightpercent has been made possible by the invention also for a continuoustransformation of the dicyandiamide whereby the transformation in thereactor may be kept thermally neutral.

By maintaining high dicyandiamide concentration constant in the reactor,high transformation temperatures are obtained with low gas pressure, atwhich temperatures the speed of transformation, and therefore thespace/time yield, may be kept on an even level.

For the purpose of maintaining high dicyandiamide concentrationsconstant in the reactor, the ammonia evaporated in the reactor may beused after condensation, manometric storage and cooling for theself-regulating dosed addition of dicyandiamide mash, ordicyandiamide-ammonia stock solution, to the reactor.

We claim:

1. In a process for producing melamine which comprises the steps offorming a solution of dicyandiamide in pure liquid ammonia wherein theconcentration of dicyandiamide is between about 44-97% by Weight of thetotal solution, maintaining said solution in a reaction zone at atemperature of about 133205 C. and at a pressure of about 41-112atmospheres, whereby melamine is precipitated out of the solution, theimprovement comprising the steps of evaporating ammonia which, afterescaping at the top of the reaction zone, is condensed to form liquidammonia at the same pressure as that prevailing in that zone and cooledto a temperature of about 25 -30 C. removing part of said liquid ammoniato form additional solution with fresh dicyandiamide to be passed intothe reaction zone as the initial solution after being preheated tosubstantially liquefy the dicyandiamide content, passing theprecipitated melamine after leaving the reaction zone at the bottom incountercurrent to a continuously upward moving stream of the rest ofsaid liquid ammonia thereby washing out and absorbing heat from saidmelamine and at the same time reconveying any unconverted dicyandiamidepresent back to the reaction zone, heating the mixture of said melamineand ammonia, leaving the reaction zone under ambient pressure in acirculatory stream of hot ammonia gas, thereby evaporating the liquidammonia and separating it from circulation as a gas, which is recycledafter condensation, separating the dry melamine, in turn, from the gasstream, and discharging said dry melamine as the end product underambient pressure, said above steps being carried out in continuousmanner.

2. The process of claim 1 wherein the solution of dicyandiamide inliquid ammonia is saturated.

3. The process of claim 1 wherein the dicyandiamide content of thesolution to be passed into the reaction zone is maintained at such ahigh concentration that the ammonia vapor pressure formed during thepreheating is lower than the pressure in said zone whereby evaporationis avoided during the preheating.

References Cited in the file of this patent UNITED STATES PATENTS2,164,705 Fi'sch July 4, 1939 2,170,491 Widmer et a1. Aug. 22, 19392,191,361 Widmer et al. Feb. 20, 1940 2,283,209 Hull et al. May 19, 19422,375,730 Caldwell et al. May 8, 1945 2,375,731 Caldwell May 8, 1945 T'I'WW.

1. IN A PROCESS FOR PRODUCING MELAMINE WHICH COMPRISES THE STEPS OFFORMING A SOLUTION OF DICYANDIAMIDE IN PURE LIQUID AMMONIA WHEREIN THECONCENTRATION OF DICYANDIAMIDE IS BETWEEN ABOUT 44-97% BY WEIGHT OF THETOTAL SOLUTION, MAINTAINING SAID SOLUTION IN A REACTION ZONE AT ATEMPERATURE OF ABOUT 133* - 205* C. AND AT PRESSURE OF ABOUT 41-112ATMOSPHERES, WHEREBY MELAMINE IS PRECIPITATED OUT OF THE SOLUTION, THEIMPROVEMENT COMPRISING THE STEPS OF EVAPORATING AMMONIA WHICH, AFTERESCAPING AT THE TOP OF THE REACTION ZONE, IS CONDENSED TO FORM LIQUIDAMMONIA AT THE SAME PRESSURE AS THAT PREVAILING IN THAT ZONE AND COOLEDTO A TEMPERATURE OF ABOUT 25* - 30* C. REMOVING PART OF SAID LIQUIDAMMONIA TO FORM ADDITIONAL SOLUTION WITH FRESH DICYANIDIAMIDE TO BEPASSED INTO THE REACTION ZONE AS THE INITIAL SOLUTION AFTER BEINGPREHEATED TO SUBSTANTIALLY LIQUEFY THE DICYANDIAMIDE CONTENT, PASSINGTHE PRECIPITATED MELAMINE AFTER LEAVING THE REACTION ZONE AT THE BOTTOMIN COUNTERCURRENT TO A CONTINUOUSLY UPWARD MOVING STREAM OF THE REST OFSAID LIQUID AMMONIA THEREBY WASHING OUT AND ABSORBING HEAT FROM SAIDMELAMINE AND AT THE SAME TIME RECONVEYING ANY UNCONVERTED DICYANDIAMIDEPRESENT BACK TO THE REACTION ZONE, HEATING THE MIXTURE OF SAID MELAMINEAND AMMONIA, LEAVING THE REACTION ZONE UNDER AMBIENT PRESSURE IN ACIRCULATORY STREAM OF HOT AMMONIA GAS, THEREBY EVAPORATING THE LIQUIDAMMONIA AND SEPARATING IT FROM CIRCULATION AS A GAS, WHICH IS RECYCLEDAFTER CONDENSATION, SEPARATION THE DRY MELAMINE, IN TURN, FROM THE GASSTREAM, AND DISCHARGING SAID DRY MELAMINE AS THE END PRODUCT UNDERAMBIENT PRESSURE SAID ABOVE STEPS BEING CARRIED OUT IN CONTINUOUSMANNER.